The invention relates to a method of creating a high-resolution still picture, comprising the steps of: receiving a sequence of lower-resolution pictures; estimating motion in said sequence of lower-resolution pictures with sub-pixel accuracy; and creating the high-resolution still picture from said sequence of lower-resolution pictures and said estimated motion. The invention also relates to an arrangement for creating a high-resolution still picture, for example, an electronic still-picture camera.
A prior-art method of creating a high-resolution still picture as defined in the opening paragraph is disclosed in European Patent Application EP-A-0 731 600. In this prior-art method, one of the lower resolution pictures is selected as a reference picture, and the relative motion between the pixels of the reference picture and each one of the other pictures is estimated with sub-pixel accuracy. Using the motion thus estimated, the lower resolution pictures are scaled to the high-resolution domain and combined to form the high-resolution picture. The relative motion is represented in the form of a mapping transform.
It is an object of the invention to provide a method of creating still pictures with advantageous effects in terms of performance and practical implementations.
To this end, the method in accordance with the invention comprises the steps of subjecting the sequence of pictures to motion-compensated predictive encoding, thereby generating motion vectors representing motion between successive pictures of said sequence; decoding said encoded pictures; and creating the high-resolution picture from said decoded pictures and the motion vectors generated in said encoding step.
The creation of a high-resolution picture from a sequence of lower-resolution pictures relies on the availability of sub-pixel motion information. Employing motion-compensated predictive encoding based on motion between successive pictures (instead of motion between each picture and a fixed reference picture) increases the probability that motion vectors with sub-pixel accuracy will be obtained. The performance of the method is thus considerably improved. The invention also has the advantage that the sequence of motion-compensated predictively encoded lower-resolution pictures is a compressed representation of the high-resolution still picture. Accordingly, the still picture can efficiently be stored and/or transmitted. Because the motion vectors are part of the stored data, the high-resolution still picture can then be obtained without necessitating another motion estimator. A further advantage is that, upon reproduction, the user may select creation of the high-resolution still picture or playback of the original lower-resolution video sequence.
Preferably, the step of encoding the sequence of pictures comprises the use of an MPEG encoder which is arranged to produce an IPPP. sequence of encoded pictures. Cost-effective MPEG encoders with high compression ratios are readily available.
In an embodiment of the invention, the high-resolution still picture is created by recursively adding a current decoded picture to a previously created picture, said previously created picture being subjected to motion-compensation in accordance with the motion vector which is associated with the current decoded picture. It is thereby achieved that the still picture is gradually built-up in a single picture memory.
The invention can also be used to create a high-resolution still picture from an already available (received or recorded) sequence of motion-compensated predictively encoded lower-resolution pictures, for example, an MPEG bitstream.